FIELD OF INVENTION
This invention relates to lugs for hanging airborne stores from an aircraft, and particularly to a clevis assembly having a folding clevis which provides longitudinal restraint of the store when the store is carried by the aircraft, and which retracts and rotates into a recess out of the airstream when the store is released from the aircraft.
"Airborne stores" are articles designed to be carried by aircraft, typically suspended from an ejector or rack secured to the underside of the wings or fuselage, often in such a manner that the store can be ejected or dropped at the desired time by triggering the ejector or rack from within the aircraft to release and eject the store. The best known type of airborne stores are munitions such as bombs and missiles, but they may also include jettisonable external fuel tanks, rescue equipment, emergency supplies, chaff, flares, and so forth.
The basic function of a lug for an airborne store is to provide an attachment point by which the store can be hung from the aircraft. It also provides a convenient "handle" for lifting and moving heavy, delicate or explosive stores on the ground, using techniques and equipment that ensure that it will not be dropped or impacted against other structure.
Another function of a lug is to enable the store to be secured against side-to-side or lateral motion while carried by an aircraft in flight. Normally, lateral restraint is provided by two sets of laterally extending sway braces which firmly engage the store at laterally spaced positions on both sides of the lugs. The lug provides the reaction lifting force to the laterally spaced pushing forces exerted by the sway braces.
Fore-and-aft or longitudinal restraint is provided, for stores with non-retracting or fixed lugs, by the vertical rigidity or torque resistance of the fixed lugs. Longitudinal restraint is important, not merely to prevent motion of the store in the fore-and-aft direction, but also to ensure that the store is released from both sets of shackles when the ejector operates. If the store were free to move to the rear when the shackles release, the line of action of one over-center shackle could shift into alignment with the lug, and the store might then hang up on the forward shackle instead of releasing cleanly. This is a very dangerous situation for the pilot of an ordnance delivery aircraft: he cannot land safely until the munition is released or secured, so he must attempt to shake the munition loose without causing it to swing violently against the aircraft. If he is unable to dislodge the hanging munition the only recourse is for the pilot to eject from the aircraft and allow it to crash. This presents an unacceptable danager to the pilot and an unacceptable risk of loss of expensive aircraft. The armed services, therefore, are keenly interested in obtaining very reliable hardware for hanging and releasing munitions.
The use of "retracting lugs" has occurred recently in the field of munitions, primarily to improve the performance characteristics of high speed maneuverable ordnance such as missiles and "smart" bombs. A lug projecting into the airstream past an airborne vehicle produces an unbalanced drag which is roughly proportional to the square of the airspeed, so the use of a "retracting lug" to avoid the adverse aerodynamic effect on high speed munitions becomes increasingly significant as the speed and maneuverability increase. (The term "retracting lug" is actually a literal contradiction since the term "lug" implies a fixed or non-retracting attachment ring or clevis. A more accurate term, and the one used hereinafter, for a folding or retracting attachment point for an airborne store is "folding clevis.")
Another important advantage of the folding clevis is its low radar cross-section. The newly emerging sophisticated defense systems against air launched munitions depend to some extent on radar acquisition and tracking, and these defenses can be nullified to the extent that the radar cross-section of the incoming munitions falls below the sensitivity of the defensive radar. A projecting lug produces a bright radar return signal; a folded clevis can be concealed in such a way as to produce little or no return signal.
The use of the folding clevis in the past has required special ejector modifications to provide the longitudinal restraint previously provided by the fixed lug. Since a folding clevis has no torque resistance in its folding direction (toward the rear or tail of the munition) and since that is the direction in which the primary longitudinal restraint must be applied during aircraft landing, a separate longitudinal restraint has been provided for prior art folding clevises in the form of a shear pin extending from the ejector into a recess in the clevis housing. The pin absorbs the longitudinal inertial forces and prevents longitudinal movement until the ejector hooks disengage from the clevises, allowing the store to drop vertically clear of the shear pin.
Although the shear pin modification has worked well, it has required modification of the ejectors, thereby increasing their size, cost and weight. It would be preferable to modify the clevis to allow it to retract freely into its pocket when released, but which would provide secure longitudinal restraint, equivalent to that of the old fixed lug, when extended.
Another technique to attain longitudinal restraint in a retractable munitions clevis uses a pair of parallel spaced-apart plate-like members, each of which are slotted to receive the clevis and engage the longitudinally facing sides thereof to prevent the clevis from rotating. This technique will prevent rotation of the clevis to the extent that clearance between the clevis and the slot is minimized. As the clearance is minimized, the danger of one clevis becoming jammed in the slot by not-quite simultaneous release of the two shackles becomes greater.